The present invention relates to a frequency-shift-keying (FSK) detector suitable for incorporation into a large-scale integrated circuit.
FSK is a modulation system that encodes digital data as shifts in a carrier frequency. Binary FSK employs frequency shifts of -.DELTA.f and +.DELTA.f to represent single bit values (0 or 1); quaternary FSK employs frequency shifts of -3.DELTA.f, -.DELTA.f, +.DELTA.f, and +3.DELTA.f to represent dibit values (00, 01, 10, or 11); and in general M-ary FSK employs frequency shifts from -(M-1).DELTA.f to +(M-1).DELTA.f to represent L-bit values, where L and M are positive integers such that M=2.sup.L.
A conventional M-ary FSK detector employs a discriminator, which is basically a bank of M bandpass filters tuned to frequencies from -(M-1).DELTA.f to +(M-1).DELTA.f at intervals of 2.DELTA.f, to demodulate the FSK signal and recover the data. The discriminator, however, is an analog circuit comprising resistors, capacitors, and inductors, so its performance depends strongly on resistance, capacitance, and inductance values, which are notoriously prone to variation due to non-uniform fabrication conditions. Its analog circuit elements are moreover difficult to implement in a large-scale integrated circuit.